1. Field of the Invention
The present invention relates generally to implantable prostheses for replacing human skeletal joints, and relates more particularly to a femoral component of an implantable total knee joint prosthesis.
2. Background Art
Implantable orthopedic prostheses, in one form, comprise manufactured replacements for the ends and articulating surfaces of the bones of the skeleton. Such prostheses are implanted to repair or reconstruct all or part of an articulating skeletal joint that is functioning abnormally due to disease, trauma, or congenital defect. Various articulating skeletal joints of the human body are eligible to be fitted with implantable orthopedic prostheses, the knee joint being one of the joints treated most often. The knee joint is a major weight bearing joint and degenerates more quickly than some other joints in case of abnormality. Also, the knee joint plays a critical role in ambulation and quality of life. Consequently, the demand is great for surgical correction of abnormalities of the knee.
The human knee joint involves three bones: the femur, the tibia and the patella, each having smooth articulation surfaces arranged for articulation on an adjacent articulation surface of at least one other bone. The femur includes at its distal extremity an articulation surface having medial and lateral convex condyles separated posteriorly by an intercondylar groove running generally in the anterior-posterior direction. The condyles join at the distal-anterior face of the femur to form a patellar surface having a shallow vertical groove as an extension of the intercondylar groove. The patella includes on its posterior face an articulation surface having a vertical ridge separating medial and lateral convex facets, which facets articulate against the patellar surface of the femur and against the medial and lateral condyles during flexion of the knee joint, while the vertical ridge rides within the intercondylar groove to prevent lateral displacement of the patella during flexion. The tibia includes at its proximal end an articulation surface having medial and lateral meniscal condyles that articulate against the medial and lateral condyles, respectively, of the femur. The mutually engaging articulation surfaces of the femur and the patella together form, functionally, the patellofemoral joint, and the mutually engaging articulation surfaces of the femur and tibia together form, functionally, the tibiofemoral joint, which two functional joints together form the anatomical knee joint.
One or more of the articulation surfaces of the knee joint may fail to act normally, requiring the defective natural articulation surface to be replaced with a prosthetic articulation surface provided by an implantable prosthesis. To fit defects of varying scope, while allowing healthy portions of the knee joint to be conserved, a range of types of orthopedic knee implants is available. The range extends from total knee prosthesis systems for replacing the entire articulation surface of each of the femur, tibia and patella, to simpler systems for replacing only the tibiofemoral joint, or only one side (medial or lateral) of the tibiofemoral joint, or only the patellofemoral joint. Commonly employed orthopedic knee prostheses include components that fall within one of three principle categories: femoral components, tibial components, and patellar components. A so-called "total" knee prosthesis includes components from each of these categories. The femoral component replaces the distal end and condylar articulating surfaces of the femur and may include a proximal stem received within the medullary canal at the distal end of the femur. The tibial component replaces the proximal end and meniscal articulating surfaces of the tibia and may include a distal stem received within the medullary canal at the proximal end of the tibia. In some designs, the proximal stem of the femoral component or the distal stem of the tibial component is optional and is provided as a modular component. The patellar component replaces the posterior side and natural articulating surface of the patella. Sometimes, the patellar component is not used, and the natural articulating surface of the patella is allowed to articulate against the femoral component. Often, two versions of a particular prosthesis are made available: a "primary" prostheses and a "revision" prostheses. The primary version of the prosthesis is generally more bone conserving than the revision version and is designed with the assumption that it will be the first prosthesis implanted in a patient's knee joint. The revision version is usually used to replace a previously implanted primary prosthesis that has failed. As compared to primary prostheses, revision prostheses replace a greater amount of bone and are usually thicker and have different geometries of the bone-engaging surface.